Variable Frequency Drives Work
AC electric motors are the workhorses of industry – cost-effective, reliable and long-lasting. They are found in everything from conveyor belt drives for manufacturing, to machine tools, pumps and fans. Yet, one thing that has been challenging to achieve with these machines is precise speed control. While AC motors can be controlled by changing the voltage they use, adjusting the frequency of the current requires more advanced technology. This is where variable frequency drive (VFD) systems come in.
VFDs are microprocessor-based electronic devices that manage the ratio of frequency and voltage to control motor speed. They are a key component in the modern industrial world and used in a wide range of applications, including powering traction trains, regulating frequency for power plants, controlling the spindle on CNC machine tools to provide greater precision, controlling elevators and escalators, and providing better speed regulation on cranes.
When variable frequency drives start an electric motor, they apply a low frequency and voltage to avoid the large inrush currents typically seen with direct-on-line starting. Then they gradually increase the frequency and voltage to accelerate the load. This method prevents the windings from heating up and damaging bearings, gearboxes, or connected equipment. It also reduces energy consumption by limiting the power drawn from the mains during acceleration and deceleration.
How Do Variable Frequency Drives Work?
There are many types of VFDs designed to meet the specific needs of different applications. They differ in the methods they employ to vary the frequency and voltage of the electric current, as well as how they deal with harmonics. The most common VFDs use a pulse-width modulation (PWM) process to generate currents that are shaped to simulate the waveform of a sinusoid. This technology gives them excellent input power factor, eliminates motor cogging below 6 Hz, and allows for regenerative power capabilities that recover and return braking energy to the power system.
Vector control VFDs are a more sophisticated type of VFD that uses a mathematical algorithm to manage the voltage/frequency relationship as the speed of the motor decreases. This enables the drives to deliver more precise speed control, up to and even beyond the base motor speed.
Across-the-line (OTL) drives are the basic type of VFD, commonly used in simple applications such as conveyor belts or small pumping systems. They are typically cheaper than vector control drives and do not offer regenerative capabilities.
Line regenerative VFDs are the most advanced type of variable frequency drives and the only ones that can recover energy for reusing in the power system. These systems have capacitors and inductors attached to filter the regenerated power and return it to the line. This is an effective way to reduce energy costs while maintaining the efficiency of the AC motor.